1. Field of the Invention
The present invention relates to a push button switch used for a personal computer, a word processor and the like, and more particularly to a push button switch which uses a membrane switch in a contact portion and has a click feeling.
2. Description of the Prior Art
A prior art push button switch of this kind is as shown in FIGS. 6 (a) and (b), which are respectively sectional views showing the state before and after the depression of a key top of a conventional push button switch.
In these Figures, a key indicated at 1 is mounted slidably along a hollow cylindrical supporting body 3 which is stood upright on a frame 2. The frame 2 has one cylindrical supporting body 3 per to one key 1 on a keyboard. The frame 2 is mounted on a base plate 4.
The key 1 has a downwardly extending mandrel 5. The mandrel 5 is located on the inside of the cylindrical supporting body 3 of the frame 2, whereby the key 1 is within the cylindrical supporting body 3, and whereby the the key 1 is slidably moved within the cylindrical supporting body 3. In the outer surface of the mandrel 5 having two bifurcated and separated skirt portions 6 (only one of which is shown) and in the inner surface of the cylindrical supporting body 3, a rib and a slot are provided so that during the movement of the key 1 in a vertical direction, the rib and slot cooperate to direct and guide the movement of the key 1.
A spring 7 is compressed between the key 1 and the actuator 8. When the key 1 is depressed, the actuator 8 of a pivotal type closes a contact switch 10 of a membrane contact switch construction 9. An upper end of the spring 7 acts on a mounting bed 11 which is present in the mandrel 5 of the key 1. The mounting bed 11 is slightly inclined so that the spring 7 is bent in a first selected direction (rightward in FIG. 6 (a)) when the key is not pressed. Buckling of the spring 7 in a lateral direction is restricted by the skirt portion 6 of the mandrel 5.
Since the actuator 8 is always at rest on the upper surface 12 of the membrane contact switch construction 9, the spring 7 always provides an upward bias the key 1. The upward movement of the key 1 is restricted by stop portions 13 (only one of which is shown). The stop portions 13 are adjacent to each skirt portion 6 of the mandrel 5 are affixed to the outer surface of the skirt portions 6, and are in engagement with shoulders 14 (only one of which is shown) on the inner surface of the cylindrical supporting body 3. Each of the stop portions 13 has an inclined surface. The inclined surface of the stop portion 13 is paired with the inclined surface of the shoulder 14 so that in the case where in assembling the keyboard or in replacing the key 1, the key 1 can be inserted into the cylindrical supporting body 3 and each stop portion 13 will be obstructed by the shoulder 14.
In the state wherein the key 1 is not depressed and is pushed up to the uppermost end by the spring 7, the actuator 8 is at rest as shown in FIG. 6 (a). In this state, surfaces in contact with the upper surface of the membrane contact switch construction 9 are only a bottom surface 16 of a supporting bed 15 of the actuator 8 and a bottom surface 18 of a supporting bed 17. The bottom surface 16 of the supporting bed 15 and the bottom surface 18 of the supporting bed 17 are sufficiently distant from a contact switch 10, when the actuator 8 is at rest. Thus, even though the spring 7 is compressed, no force is applied to the contact switch 19. When the actuator 8 is at rest, there is a slight clearance between a bottom surface 21 of a projection 20 and the upper surface 12 of the membrane contact switch construction 9.
The membrane contact switch construction 9 has an upper layer 22, an intermediate layer 23 and a lower layer 24. The upper layer 22 has a bottom surface 25 formed with circular contacts 26 (only one of which is shown). The lower layer 24 has an upper surface 27 formed with a circular contact 28 (only one of which is shown). The intermediate layer 23 is formed with a circular opening 29. These circular contacts 26 and 28 constitute a contact switch 10. Accordingly, when the upper layer 22 is moved into the circular opening 29 of the intermediate layer 23, the circular contact 26 and the circular contact 28 contact to close the contact switch 19, as shown in FIG. 6 (b).
When the key 1 is depressed from the FIG. 6 (a) position to the FIG. 6 (b) position, force F transmitted by the spring 7 to the actuator 8 is increased by the force applied to the key 1. When the force F becomes greater than the force when the actuator 8 is at rest, moment M becomes larger than a moment formed by a product of the force F and a distance x. The moment M is produced when the spring 7 is excessively buckled from the FIG. 6 (a) position to the FIG. 6 (b) position to increase the buckling amount thereof. The actuator 8 is rotated about an initial pivot, that is, about fore ends 30 and 31 of the bottom surface of the supporting bed. By this clockwise rotation, the bottom surface 21 of the projection 20 comes into contact with the upper surface 12 of the membrane contact switch construction 9. This forces the upper layer 22 of the membrane contact switch construction 9 to move into the circular opening 29 of the intermediate layer 23. Accordingly, the circular contact 26 of the bottom 25 of the upper layer 22 comes into contact with the circular contact 28 of the upper surface of the lower layer 24.
When the actuator 8 is rotated about the initial pivot, the bottom surface 21 of the projection 20 of the actuator 8 comes into contact with a point 32 of the upper layer 22 of the membrane contact switch construction 9.
When the contact switch 10 is closed, an user feels this by the excessive buckling (FIG. 6 (b)) of the spring 7, and thus releases his finger from the key 1.
In the aforementioned prior art, the spring 7 provided between the key 1 and the actuator 8 is buckled by the downward movement of the key 1 to obtain a click feeling. Since the buckling of the spring 7 is due solely to longitudinal forces on the ends of the spring, the timing of the buckling can vary as properties of the spring vary within manufacturing tolerances. This variation in timing can result in unstable switching and unstable tactile feedback for an operator. Further, there is a limit as to how short the spring can be for a given set of parameters if buckling is to be produced by longitudinal forces applied to the spring. This problem limits the compactness of the switch.